The present invention relates to a method and an apparatus for bending a glass sheet.
A curved glass sheet has been utilized as a windowpane of an automobile. As methods for bending a glass sheet, there have been several sorts of methods, such as press bending and bending by gravity. A glass sheet for the windshield of an automobile has been prepared as stated below. The glass sheet has a peripheral portion of a lower surface supported by a four sided and ring shaped mold having a rectangular or trapezoidal shape substantially conforming to the peripheral portion of the glass sheet, and the glass sheet is heated at a temperature to be bent by gravity.
During the bending operation, it is important (1) to cool the peripheral portion of the glass sheet more rapidly than an inside portion of the glass sheet to maintain the strength at the peripheral portion and (2) to facilitate to bend the glass sheet deeply. Heretofore, a first mold and a second mold combined as a dual frame structure have been used to transfer the glass sheet from the first mold onto the second mold in the course of the bending operation.
Explanation of item (1) will be made. A laminated glass sheet for a windshield is mounted to an automobile body by an adhesive applied to a peripheral portion of the glass sheet. From this viewpoint, the peripheral portion of the glass sheet needs to have sufficiently high strength. In order to obtain the required strength, it is necessary to form a compressive stress layer by natural cooling after having heated the glass sheet. In this case, in order to improve the shaping accuracy of the glass sheet and to sufficiently bend portions of the glass sheet close to edges thereof, it is preferable that the glass sheet is supported only at the edges thereof.
However, when the glass sheet is supported at the edges thereof, a tempered layer is difficult to be formed since the edges are not cooled in a sufficient fashion because of the heat capacity of the molds. In order to cope this problem, the glass sheet is transferred onto the second mold so as to have portions slightly more inward than the edges supported. In other words, the glass sheet has the edges or portions close to the edges (hereinbelow, referred to as the edge portions) supported by the first mold during bending by gravity, and then the glass sheet has portions more inward than the edge portions supported by the second mold. The technique of transference between the molds has been disclosed in, e.g., JP-A-2-30632.
Next, explanation of item (2) will be made. When deep bending or forming a complicated shape is carried out, bending by gravity is made by supporting a glass sheet by a first mold for shallow bend and then transferring the glass sheet onto a second mold for deep bend in the course of the bending operation. It is possible not only to increase the shaping accuracy of, in particular, the corners of the glass sheet but also to improve the quality of the shaped surface by using the two molds having different curvatures to bend the glass sheet in a two-stage fashion by gravity. The technique of transference between the molds for facilitating the bending operation has been disclosed in, e.g., Japanese Patent No. 2889877.
However, in the molds disclosed in each of the two publications, the second mold is provided inside the first mold, and the glass sheet is separated from the first mold during transference by thrusting up the second mold inside the first mold or dropping the first mold. As a result, the first mold and the second mold adjacent thereto move, passing each other in a vertical direction while being adjacent each other.
However, a mechanism for lifting or lowering either one of the first and second molds to make the transference of the glass sheet is complicated. In particular, when the mechanism is configured to thrust up the entire second mold, the bending apparatus has a large size and becomes heavier. This creates a problem that the apparatus needs a driving mechanism having a great deal of output and much consumption electric power for the bending operation.
On the other hand, when the mechanism is configured to move both molds so as to pass each other, the first mold and the second mold need to have a space provided therebetween to prevent both molds from interfering each other. Because of the provision of the space, the second mold needs to be provided in a quite inner side. When the glass sheet is supported in that state, the glass sheet is apt to have mold impression of the second mold marked on an inner side of the glass sheet.
The impression exposed on the front surface of a windowpane is not preferable since it harms the appearance of the product. Although the second mold needs to be located at a position more inward than the peripheral portion of the glass sheet in order to maintain the strength of the peripheral portion at a sufficient level, the second mold is preferably located at a position close to the first mold in the order of mm thereinside since the provision of the second mold at a quite inner side creates impression as stated earlier. The conventional arrangement for making the transparence from the first mold onto the second mold has been difficult to realize it.
The present invention is proposed in consideration of the problems of the prior art. It is an object of the present invention to provide a method and apparatus for bending a glass sheet capable of restraining the creation of mold impression, which is apt to be marked when a glass sheet is transferred from a first mold onto a second mold.
The present invention provides a method for bending a glass sheet comprising using a first mold for supporting a peripheral portion of a glass sheet and a second mold provided inside the first mold; heating the glass sheet put on the first mold to a temperature to bend the glass sheet by gravity; and then transferring the glass sheet onto the second mold to bend the glass sheet only by the second mold, characterized in that the method comprises moving the first mold in a direction to spread the first mold laterally with respect to the second mold for the transference of the glass sheet from the first mold onto the second mold.
In a preferred mode of the present invention, it is preferable that the method further comprises supporting an edge of the glass sheet by the first mold.
In a preferred mode of the present invention, it is preferable that the method further comprises making the transference of the glass sheet from the first mold on the second mold by rotating the first mold about a rotary shaft.
In a preferred mode of the present invention, it is preferable that the method further comprises making the transference of the glass sheet from the first mold on the second mold by moving the first mold in a direction tangential to a surface of the glass sheet close to an edge thereof.
In a preferred mode of the present invention, it is preferable that the glass sheet is utilized as a windowpane for an automobile.
The present invention also provides an apparatus for bending a glass sheet comprising a first mold supporting a peripheral portion of a glass sheet and a second mold provided inside the first mold; wherein the glass sheet put on the first mold is heated to a temperature to bend the glass sheet by gravity, and then the glass sheet is transferred onto the second mold to bend the glass sheet only by the second mold; characterized in that the first mold includes divided sections for respective sides, the divided sections being configured to move in directions to spread laterally with respect to the second mold.
In a preferred mode of the present invention, it is preferable that the second mold is formed in a ring shape.
In a preferred mode of the present invention, it is preferable that the first mold comprises four bar-shaped member provided along respective four sides.
In a preferred mode of the present invention, it is preferable that the first mold comprises two bar-shaped member provided so as to confront each other.
In a preferred mode of the present invention, it is preferable that each of the bar-shaped members is shorter than a length of one side of the glass sheet.
In a preferred mode of the present invention, it is preferable that the first mold has a substantially L-character shape in section and an upper flange inwardly projected in a reversed L-character shape, and the second mold is provided under the L-character shaped upper flange.
In a preferred mode of the present invention, it is preferable that the glass sheet is utilized as a windowpane for an automobile.